[TowerTalk] Shunt Feeding a Vertical

[jimlux]

On 9/5/19 1:57 PM, David Gilbert wrote:
> 
> Shunt feed essentially involves building a transmission line comprised 
> of the tower and the shunt wire.  Your 50 feet equates to not much more 
> than 2% of a wavelength at 475 KHz, so yeah ... it sounds to me like 
> you're pretty much just trying to feed a shorted transmission line.

exactly.. wavelength at 475k is 631 meters (about 2100 ft), and your 
tower is 325 ft, about 1/8th wavelength.

If it's an AM BC tower, with an isolated base, maybe you can just feed 
it at the base as a "short monopole" with an appropriate LC matching 
network.

the Z will be about 11 -157j ohms, so that's not too bizarre to match.



> 
> 73,
> Dave   AB7E
> 
> 
> 
> On 9/5/2019 1:34 PM, Roger Parsons via TowerTalk wrote:
>> I thought I understood shunt feeding, but I clearly do not. I am 
>> pretty au fait with determining gamma matching parameters, but for 
>> this application I want generic shunt feed.
>>
>> I have use of a 325' grounded former AM broadcast tower (on which I 
>> have various 160m antennas). I now want to feed it on 475kHz as well. 
>> I had been of the opinion that one could tap at any point on the 
>> tower, bring a wire down from that point, and end up with some 
>> impedance which could then be matched. Whilst that may be generally 
>> true, there is clearly more to it. My first attempt was a tap at 50', 
>> spaced about 18' from the tower. The impedance measured at the bottom 
>> of the wire was almost zero - that is it appeared as if it was the 
>> tower itself, and that was not useful.
>>
>> As gamma matching clearly works, and as other people have successfully 
>> used shunt feeding, it must be possible. It has been suggested that I 
>> should go higher up the tower (say 100'), and space the wire further 
>> out. I could do this, but would like some confirmation that it is 
>> likely to work first.
>>
>> 73 Roger
>> VE3ZI
>>
>>
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